The present invention relates to electrophotography and more particularly, to a dry process developing apparatus for use in an electrophotographic copying machine.
Conventionally, for dry process developing employing developing materials containing magnetizable components, there has been proposed, for example, in Japanese Laid Open Patent Application Tokkaisho No. 52-17831, a developing apparatus of the rotatable outer cylinder or developing sleeve type in which a multipolar magnet member is fixedly disposed in a rotatable developing sleeve of non-magnetizable material and developing material transported onto the developing sleeve is formed into a magnetic brush on the peripheral surface of the developing sleeve, the amount being restricted by a doctor blade for developing an electrostatic latent image formed on a photosensitive member or photoreceptor into a visible image. There has also been proposed, in Japanese Laid Open Patent Application Tokkaisho No. 52-67336, a developing apparatus of the rotary magnet type in which the developing material is formed into the magnetic brush on the outer periphery of a fixed developing sleeve of non-magnetizable material by magnetic force from an alternating field produced by a rotary multipolar magnet member rotatably housed in the fixed outer sleeve for developing the electrostatic latent image formed on the photoreceptor by the magnetic brush thus formed.
However, since the developing materials employed in the known developing apparatuses as described above have extremely fine particles, such developing materials tend to be aggregated or collected into lumps due to moisture and electrostatic attraction between the particles of the developing material, and the developing material supplying port or doctor blade portion for regulating the amount of the developing material on the developing sleeve becomes clogged by these lumps, thereby obstructing smooth transfer of the developing material and resulting in disadvantages, for example, deterioration of the quality of the copied image due to fogging by the toner.
Meanwhile, owing to the fact that both of the conventional developing apparatuses, the rotatable developing sleeve type and the rotary magnet type, as described above are arranged to effect development by lightly rubbing the developing material in the shape of a magnetic brush formed on the developing sleeve against the electrostatic latent image on the photoreceptor, the density of the developed images is proportional to the speed of movement of the developing material over the developing sleeve. More specifically, in order to obtain sufficient density of the developed images, it is necessary that the developing material moving on the surface of the developing sleeve in the form of the magnetic brush rubs against the surface of the photoreceptor bearing the latent image formed thereon at a comparatively high speed. Although the speed of movement of the developing material is sufficiently high in the developing apparatus of the rotating developing sleeve type as described above, the developing material tends to be solidified due to compression between the developing sleeve and the photoreceptor, thus resulting in insufficient developing treatment. In the developing apparatus of the rotary magnet type in which the multipolar magnet member is rotated at a very high speed, eddy currents are produced on the developing sleeve, which is normally made of non-magnetizable electrically conductive material (for example, aluminum) through rotation of the multipolar magnet member disposed therein, with the phenomenon becoming conspicuous as the speed of rotation of the magnet member increases, giving rise to adverse effects such as temperature rise of the developing sleeve itself, increase of driving torque needed for rotating the multipolar magnet member, etc.
For improving the image density without rotating the developing sleeve or multipolar magnet member so as to eliminate the temperature rise, increase of the driving torque, etc. as described above, it has been suggested to increase the length of contact between the developing material and surface of the photoreceptor having the electrostatic latent image defined thereon by forming a developing material collecting zone as indicated at Q in FIG. 5 by feeding of a sufficient amount of developing material onto the developing sleeve. The arrangement as described above, however, still has a disadvantage in that there is produced a zone P in the developing material collecting zone Q at a position spaced somewhat from the developing sleeve 11 and which is not affected by the magnetic force of the multipolar magnet member 12, in which zone P, the developing material is solidified or remains stagnant so as to give rise to fogging and adversely affecting the quality of the copied images.
In the case where toner particles of an electrically insulating nature are employed for developing electrostatic latent images formed on the photoreceptor in electrophotographic copying machines in general, the machine is normally so arranged that the developing material including toner particles and magnetizable carrier particles is stirred for causing the toner particles and magnetizable carrier particles to uniformly contact each other, but in the conventional developing apparatuses, it has been rather difficult to fully stir the developing material completely due to deficiencies in the construction of such known developing apparatuses. For eliminating disadvantages in the conventional dry process two-component developing method and one component developing method, there has been proposed by the present inventors in Japanese Patent Application No. 158110/51 a novel dry method developing process which employs as developing material a mixture prepared by mixing magnetizable toner particles having average particle diameters of 3 to 30 .mu.m and a resistivity of 10.sup.3 to 10.sup.16 .OMEGA..cm with electrically insulating non-magnetizable toner particles having average particle diameters of 3 to 30 .mu.m at a mixing ratio of 1 to less than 2.5 by weight. While the known developing method as described above is advantageous in that the image quality is not caused to deteriorate by the variation of the mixing ratio of the two kinds of toner particles and facilitated fixing, sufficient stirring of the developing material cannot be effected by the conventional stirring devices, since the non-magnetizable toner particles and magnetizable toner particles are very fine and have an extremely small mass, thus resulting in uneven charging of the toner particles which causes deterioration of the copied images, and because of the small magnetic retaining force due to the extremely small particle diameters of the magnetizable toner particles, it has been difficult to properly feed the developing material onto the developing sleeve by magnetic force in the conventional developing apparatuses.
Furthermore, in a developing apparatus for causing the non-magnetizable toner and magnetizable toner to simultaneously adhere to the electrostatic latent image to be developed as described above, it is necessary to provide a developing material supplying device or toner dispenser capable of constantly replenishing the magnetizable toner as well as the non-magnetizable toner at a rate corresponding to the variation of the electrostatic charge pattern of the electrostatic latent images to be developed.
Conventionally, as a supplying device for the developing material of the above described kind, it has been proposed to use an arrangement wherein the developing material prepared by mixing the non-magnetizable toner particles with magnetizable toner particles at a predetermined ratio in a so-called premixed state is accommodated in a developing material supplying container for simultaneously supplying the non-magnetizable toner and magnetizable toner into the developing apparatus.
In the known arrangement as described above, however, the rate of consumption of the magnetizable toner and non-magnetizable toner is constantly varying due to variations of the electrostatic charge pattern in the actual developing. More specifically, it has been confirmed by the present inventors that in ordinary originals the pattern of which is mainly composed of letters, characters, etc. (so-called line copy), the ratio of the magnetizable toner t.sub.m to non-magnetizable toner t.sub.n consumed in the developing, which is represented by (t.sub.n /t.sub.m +t.sub.n).times.100 weight %, is approximately 85 weight %, while in originals having much larger area images including photographs and the like (so-called solid copy), the ratio is approximately 95 weight %. Therefore, when developing material prepared by pre-mixing the magnetizable toner and non-magnetizable toner, for example, at a ratio of 85 weight % is loaded in the known developing material supplying container as described above for feeding, if solid copy is continuously developed, the rate of use the magnetizable toner for the developing material in the developing apparatus tends to increase as compared with the initial setting (the rate of the non-magnetizable toner for the developing material in the developing apparatus is normally at 10 weight %), resulting in difference of image quality arising from variations of density between copied images, although no particular inconveniences are encountered in the line copy.